The invention relates to extreme pressure lubricating oils, particularly alkali metal borate-containing lubricants.
Alkali metal borates are well known in the lubricant industry for their usefulness as extreme pressure agents. See, for example, U.S. Pat. Nos. 3,313,727; 3,565,802; 3,819,521; 3,846,313; 3,853,772; 3,907,691; 3,912,639; 3,912,643; 3,912,644; 3,997,454; and 4,089,790.
U.S. Pat. No. 4,459,215 discloses a lubricating composition containing an alkali metal borate, a sulfur-containing compound and a zirconium salt.
U.S. Pat. No. 4,575,431 discloses a lubricating oil containing a mixture of phosphates, said phosphates being essentially free of monothiophosphates.
U.S. Pat. No. 4,089,790 claims a synergistic lubricant mixture containing: (1) a hydrated potassium borate; (2) an antiwear agent selected from (a) zinc dihydrocarbyl dithiophosphate, (b) C1 to C20 ester, C1 to C20 amide or C1 to C20 amine salt of a dihydrocarbyl dithiophosphoric acid, (c) zinc alkyl aryl sulfonate, and (d) mixture thereof; and (3) oil soluble antioxidant organic sulfur compound.
U.S. Pat. No. 4,171,268 claims lubricant compositions containing a zirconium salt of a carboxylic acid and oil-soluble sulfur-containing extreme pressure agent.
U.S. Pat. Nos. 4,563,302 and 4,204,969 disclose sulfurized olefins useful in lubricating oils.
U.S. Pat. No. 4,717,490 to Salentine discloses a lubricating composition that is a combination of alkali metal borates, sulfur compounds, phosphites, and >50% neutralized acidic phosphates. However, this composition suffers from a shortened shelf life compared to other commercially available lubricants, which do not use solid dispersions of alkali metal borates. In particular, this composition will exhibit additive “dropout” over time. The problem becomes more severe as the storage temperature increases. The standard remedy in the industry is to add more dispersant or detergent additives to the composition to improve the shelf life. However, these additives can negatively impact other performance properties of the gear lubricant. It is, therefore, an object of the present invention to provide an alkali metal borate-containing lubricant which has superior load carrying properties and improved storage stability.
Without being bound to any specific theory, we have found that a major cause of additive dropout is the use of the dialkyl hydrogen phosphite, which is disclosed as an essential component of the combination in the Salentine patent. This material is acidic and unstable, and it appears to react with either the borate particles or with the basic dispersant and/or detergent additives that are used to stabilize the borate particles to generate a precipitate which settles to the bottom of a lubricant container or package. The acidity derives from a hydrogen that is either directly attached to a phosphorus or attached to a hetero atom which is in turn attached to a phosphorus. The present invention involves replacing the dialkyl hydrogen phosphite of the Salentine patent with a trialkyl phosphite. The trialkyl phosphite is not as reactive and the storage stability of the resultant composition improves unexpectedly and dramatically.
The Salentine patent claimed that there was a synergistic load-carrying effect obtained from combining four components. This improved load-carrying effect is preserved even when the dialkyl hydrogen phosphite is replaced by trialkyl phosphite in accordance with the present invention.
Another benefit of a composition in accordance with the present invention is that it is much easier to manufacture compared to the compositions disclosed in the Salentine patent. Dialkyl hydrogen phosphite is a solid material and is very difficult to handle during blending at the plant. Because of its reactivity and sensitivity to water, full containers of the chemical must be used. On the other hand, trialkyl phosphite is liquid at room temperature and so blending is very easy. It also exhibits much less reactivity to water.